The detection and classification of digitally modulated signals in a broad band without knowledge of their carrier frequency, modulation type, or modulation rate has long provided a difficult problem for receiver designers. Energy detection receivers such as crystal video detectors, scanning receivers, filter bank or channelized receivers, and spectrum analyzers have been the only practical type of receivers available for such use. However, they are severely limited in their ability to detect and classify specific modulation types and rates, and their performance degrades rapidly when more than one signal is present at the input.
The advantages of digital modulation in communication and radar have long been recognized. Now that the era of practical implementation and deployment of such systems has arrived, an effective detection and classification receiver is essential. Current receiving systems, designed for use against conventional signals, must be expanded to cope with digital modulation in order to maintain their effectiveness.
Until now, it has been virtually impossible to detect and classify a noncooperative or low-level digitally modulated transmission for which the key modulation parameters and center frequency are unknown. In detecting noncooperating transmitters, it is essential to
a. Detect digital modulation embedded in dense RF backgrounds. PA0 b. Classify the modulation format as Constant Phase (e.g., phase shift keying), Linear Phase (e.g., multifrequency shift keying), Quadratic Phase (e.g., linear FM or chirp), higher-order phase modulation, or their hybrids. PA0 c. Discriminate against other transmissions, including those (1) coexisting at the same frequency and (2) having similar modulation formats and clock rates. PA0 d. Significant immunity to narrowband interference. PA0 e. High-signal-to-noise-ratio signal output. PA0 f. High-signal-to-noise-ratio signal output that is fully coherent with the digital-modulation symbol transitions. PA0 g. A real-time, adaptive detection threshold. PA0 1. A. Viterbi and J. Omura, Principles of Digital Communication and Coding, McGraw-Hill, New York, 1979. PA0 2. H. Van Trees, Detection, Estimation, and Modulation Theory, Wiley, New York, 1971. PA0 3. R. C. Dixon, Spread Spectrum Systems, Wiley, New York, 1976. PA0 4. C. E. Cook et al., "Special Issue on Spread Spectrum Communications," IEEE Transactions on Communications, Vol. COM-30, No. 5, May 1982. PA0 5. W. C. Lindsey and M. K. Simon, Telecommunication Systems Engineering, Prentice-Hall, Englewood Cliffs, N.J., 1973. PA0 6. N. F. Krasner, "Optimal Detection of Digitally Modulated Signals," IEEE Transactions on Communications, Vol. COM-30, No. 5, May 1982. PA0 7. C. W. Helstrum, Statistical Theory of Signal Detection, Pergamon, Long Island City, N.Y., 1975. PA0 8. M. Skolnik, Introduction to Radar Systems, 2d ed., McGraw-Hill, New York, 1980. PA0 9. A. Papoulis, The Fourier Integral and Its Applications, McGraw-Hill, New York, 1962.
It is also important that any such receiver exhibit
The state of the art in digital modulation techniques is best described in the following publications:
The fundamental characteristic of all digital modulation techniques is the conversion of the original information into a unique set of discrete symbols drawn from a finite alphabet. The set of symbols is then used to modulate a carrier sequentially in time according to the transmitter clock, and with a well-defined symbol duration.
The state of the art in digital-modulation detection techniques (in which the receiver does not know one or more of the key characteristics of the transmitted signal) is best summarized in the following publications:
Other publications relevant to the following discussion are